1. Field of the Invention
The present invention relates to a method for characterizing the planarizing properties of a combination of expendable materials in a chemical-mechanical polishing (CMP) process, according to which a substrate that is to be polished, specifically a semiconductor wafer, is pressed onto a softcloth and rotated relative to the cloth for a defined polishing time.
The invention also relates to a method for characterizing and simulating a chemical-mechanical polishing process and a method for the chemical-mechanical polishing of a substrate, namely a semiconductor wafer.
Chemical-mechanical polishing is a method of planarizing or polishing substrates, which is common particularly in semiconductor fabrication. The advantage of planarized surfaces is that a subsequent exposure step can be carried out with a higher resolution, because the required depth of focus is smaller because of the reduced surface topography.
The basic problem in this respect is that different densities and spacings of features in the layout of a semiconductor chip influence the planarizing properties of the CMP process. Unfavorably selected processing parameters then lead to a large variation in layer thickness across the chip surface subsequent to the CMP process (global topography). On the other hand, an unfavorably selected circuit layout leads to insufficient planarizing. The insufficient planarizing impairs the follow-up processes and thus the product characteristics, because of the associated variations in layer thickness across the chip surfacexe2x80x94that is to say, across the image field surface of a subsequent exposure step. In particular, the processing window of a subsequent lithography step shrinks because of the reduced depth of focus.
Another problem in CMP is that the polishing result is influenced by a number of interacting processing parameters. Hitherto, the adjustable processing parameters, such as the rotational velocities of the polishing disk and substrate holder, the pressure, the polishing time, the quality of the softcloth, the selection of the polishing agent, or the polishing agent flow, have usually been individually adjusted for each new layer that is polished on the semiconductor wafer and for almost every new product. The optimal parameters are typically determined by trial and error in a series of test sequences. These experiments require an appreciable expenditure of time and money, as well as the presence of a sufficient number of wafers of a new product layout. The polishing agent has a mechanical and chemical erosion property (slurry).
It is accordingly an object of the invention to provide a method for characterizing the planarizing properties of a selected expendable material combination in a chemical-mechanical polishing process which overcomes the above-mentioned disadvantages of the prior art methods of this general type.
It is another object of the invention to provide a method with which the polishing result of a CMP process can be characterized more simply, and particularly to provide a method in which the number of independent parameters that must be taken into account can be reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for characterizing the planarizing properties of a combination of expendable materials in a chemical-mechanical polishing (CMP) process, whereby a substrate that will be polished, particularly a semiconductor wafer, is pressed onto a softcloth and is rotated relative to the wafer for a specified polishing time. The method includes the following steps: a) providing a combination of expendable materials including a softcloth and a polishing agent; b) prescribing a respective value range for the processing parameters of pressure (p) and relative rotational velocity (v) of the substrate and the softcloth; c) providing test substrates with test patterns with different feature densities; d) for each of the provided test substrates, prescribing a combination of values for the processing parameters of pressure and relative rotational velocity of the substrate and softcloth; e) performing a polishing process for each of the test substrates while the respective combination of values for the processing parameters is maintained until saturation is achieved; f) determining a characteristic quantity for the global grade level from the polished test substrates; and g) determining expendable material parameters that characterize the planarizing properties for the selected expendable material combination from the functional relationship between the characteristic quantity for the global grade level and the quotient of the relative velocity and pressure for each of the test substrates.
The inventive method has the advantage that an experimental characterizing only has to be performed once for a given expendable material combination, and namely is performed on a test substrate including test patterns with various feature densities. The results of characterizing the test substrate serve for determining expendable material parameters that can exhaustively describe the planarizing properties of this expendable material combination.
This makes it possible to compare the planarizing properties of different expendable material combinations with one another or to simulate polishing results with other polishing parameters and new layouts.
The test substrates provided in step (c) expediently contain line patterns with a period between 100 and 500 xcexcm, particularly of 250 xcexcm, and increasing feature densities, preferably in the range from 4% up to 72%.
In a preferred development of the method, the filter length FL is determined in step (e) as the characteristic quantity for the global grade level. The filter length, which is defined by Stine (B. Stine et al, xe2x80x9cA Closed-Form Analytic Model For ILD Thickness Variation in CMP Processesxe2x80x9d, CMP-MIS Conference, Santa Clara, Calif., February 1997), describes a window with a characteristic quantity FL over which an average is formed in a manner suitable for obtaining effective feature densities from concrete feature densities.
For instance, an averaging of the concrete feature densities can occur in the model calculation with a two-dimensional Gaussian distribution of a half-width FL. But other weight functions are also appropriate filters, for instance quadratic, cylindrical and elliptical weight functions. The elliptical and Gaussian weight functions exhibit the smallest error according to the present state of knowledge and are therefore preferable.
In a preferred development of the method, in step (f) two characteristic expendable material parameters are determined from a linear relationship between the filter length FL and the quotient of the relative velocity v and pressure p.
The slope MI and the axis segment FixFL of the fit line are expediently determined as characteristic expendable material parameters from the following linear relation:
FL(v/p)=MI*(v/p)+FixFL.
The fit line can be determined by linear regression. The two quantities MI (mechanical influence) and FixFL (a constant offset of the filter length) are then sufficient for characterizing the selected softcloth/polishing agent combination in an unambiguous fashion.
An inventive method for characterizing and simulating a chemical-mechanical polishing (CMP) process, whereby a substrate that will be polished, namely a semiconductor wafer, is pressed onto a softcloth and rotated relative to it for a defined polishing time, includes the following steps: determining layout parameters of the substrate that will be polished; prescribing a requirement profile for the CMP process result for the substrate that will be polished; specifying an expendable material combination including a softcloth and a polishing agent; characterizing the planarizing properties of the specified expendable material combination according to the method that was described above; prescribing a set of respective values for the processing parameters of the pressure (p) and the relative rotational velocity (v) of the substrate and softcloth; simulating the CMP process result for the substrate that will be polished by using the specified values for the processing parameters in connection with the previously specified characterizing expendable material parameters for determining the required polishing time; and evaluating whether the CMP process result satisfies the prescribed requirement profile.
Utilizing the above-described characterizing expendable material parameters makes a particularly effective simulation of the CMP process result possible.
The invention further provides a method for the chemical-mechanical polishing of a substrate, particularly a semiconductor wafer, whereby a CMP process is simulated with the method. A layer that will be planarized is deposited on a substrate, and the substrate is polished for a polishing time derived from the simulation. This has the additional advantage that it is unnecessary to perform a new experimental test sequence for each new substrate layout. Rather, the results of an experimental characterization of the test substrate can be utilized for the meaningful simulation and subsequent polishing of a number of various product layouts.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in method for characterizing the planarizing properties of an expendable material combination in a chemical-mechanical polishing process; simulation technique; and polishing technique, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.